I was under the impression that when a wheel is balanced properly, there is an 'outside' and 'inside' element. I've seen
some machines that just highlight what is needed at a certain point round the wheel, but others that have two scales marking weight position across
the wheel as well as around it. And it might just be me but wheels seem 'better' when they're done on a double scale machine.

You should still be able to move them slightly out of position and still have the overall balance unaffected though. And even with the most obscenely
unbalanced wheel I've only ever needed a single layer of weights (even if the strip of weights is about six inches long!!).

Edit: A quick Google and a random wiki page suggests it's the difference between a static and dynamic balance. A dynamic balance being the
'two scale' machines.

I had a similar issue years back on my hillclimb car, i just ripped off the weight that was hitting the calliper and moved it further inside the
wheel, stuck some gaffer tape over it and never noticed any difference, that was on a front wheel as well.